The Slendro scale is actually pentatonic, although it is numbered up to "6". (The European scale is heptatonic, like the Pelog scale.) There is no note "4", for some reason--once again, I'm not sure why.

What does it sound like?

Discordant, complex, rich, and extremely unique. A bit of advice--and I'm dead serious--bring ear plugs if you ever go to hear a Gamelan. It can deafen you otherwise, and you'll still be able to hear it.